The present invention relates to a method of processing a drilled roll which is used for rolling thermoplastic materials such as vinyl, rubber and the like.
The drilled roll is usually made of a chilled roll material. The outer peripheral part of the drilled roll is constituted by white cast iron while the inner part of the roll is constituted by gray cast iron. A plurality of axial bores are formed in the peripheral portion of the roll under the roll surface at a constant circumferential pitch. These axial bores are communicated with a central bore 2 through radial conduits. In operation, a high temperature heat transfer medium such as hot water is circulated through the central bore, radial conduits and then through the axial bores to heat the roll body up to a temperature of, for example, 100.degree. to 300.degree. C. which is high enough to plasticize the rolled material. During the rolling, the precision of the roll directly affects the precision of the rolled products. It is, therefore, essential to maintain a high precision of the roll during the rolling operation.
The roll, however, often exhibits a deflection when heated to the operating temperature, due to a thermal distortion, even if precisely the roll may be finished at the room temperature. Clearly, this deflection of the roll causes a fluctuation of the thickness of rolled products.
The current demand for reduced thickness and increased hardness of the rolled products goes quite contrary to the fluctuation of thickness.
The deflection of the roll at the operating temperature is an elastic deformation and, therefore, is extinguished as the roll is cooled down to the room temperature. The generation of deflection is attributable to an uneven distribution of the thermal expansion over the cross-section of the roll.
Conventionally, various measures have been taken to prevent the deflection from the view point of quality of the roll. For example, it has been attempted to uniformalize as much as possible the thickness of the peripheral layer of white cast iron, as well as the extent of the graphitization of the gray cast iron constituting the inner part of the roll. All of these measures, however, are not satisfactory.
Measures have been taken also from the view point of roll operation. For instance, it has been proposed to synchronize the rotations of the upper and lower rolls in such a manner as to negate the deflections of these rolls. It has been attempted also to reduce the absolute value of deflection by imparting an eccentricity to the bearings. These measures are also unsatisfactory to sufficiently suppress the thickness fluctuation of the products.
As a fundamental measure, it has been proposed to finish the roll by machining while maintaining the roll at the high operating temperature. This solution, however, involves various problems such as difficulty in the temperature control, difficulty in maintaining the precision of the machine tool at the elevated temperature, unknown factor of reduction of strength of the grinding wheel at the elevated temperature and so forth. For these reasons, this solution cannot be put into practical use.